The present disclosure relates to semiconductor memory devices. More particularly, the present disclosure relates to-a delay-locked loop (DLL) circuits for generating transmission core clock signals used in semiconductor memory devices.
Rapid development in semiconductor technology has brought about successful developments for digital systems such as personal computers, Portable Digital Assistants (PDA) or portable communication systems. However, an operating speed of peripheral devices could not overtake an operating speed of microprocessors in spite of an improved transmission rate of data and an improved operating speed of peripheral devices such as memories and communication devices. There has been a speed difference between new microprocessors and their peripheral devices or graphic devices, thus a remarkable improvement of speed on peripheral devices is required in high-performance digital systems.
One solution to improve speeds of peripheral devices is to add a synchronous interface to the peripheral devices. Synchronous memory devices such as synchronous DRAM or SRAM can be provided as typical examples of the synchronous peripheral devices.
In particular, in a synchronous semiconductor memory device employing a double data rate (DDR) system, an output data window of such a memory device is getting smaller according to an increasingly increased signal speed. Respective components constituting a semiconductor memory device are required to output a source synchronous clock such as an echo clock in order to span a data window; and to obtain a precise data sampling, the source synchronous clock is required to position on a center of output data and to become 50% in duty ratio.
To control a precise transmission and/or reception of source clock signal and data, a transmission core clock signal having four phases is needed. In general a phase-locked loop (PLL) or delay-locked loop (DLL) circuit is needed to generate core clock signals having such four phases. The phase locked loop circuit is sensitive to jitter of the input clock signal, is greatly affected by internal noise, and is unstable as compared with the delay-locked loop circuit. The delay-locked loop circuit is thus widely used for synchronous semiconductor memory devices.
FIG. 1 is a timing diagram schematically illustrating a transmission/reception of data by using a transmission core clock signal in a semiconductor memory device of a DDR system. Referring to FIG. 1, first data transmitted and received through a data pad DQ starts on the transmission/reception in response to a first core clock signal K<0>. An echo clock CQ,CQ\ is generated in response to a second core clock signal K<1>, which is later by 90 degrees in phase than the first core clock signal K<0>. At this time the center of the data is placed at a rising edge or falling edge of the echo clock CQ,CQ\.
A transmission/reception of second data is begun in response to a third core clock signal K<2> that is later by 90 degrees in phase than the second core clock signal K<1>. A center of the second data is placed on a rising edge or falling edge of the echo clock CQ,CQ\ in response to a fourth core clock signal K<3> that is later by 90 degrees in phase than the third core clock signal K<2>. Thus, a sampling of data can be obtained precisely. To obtain such precise sampling and transmission/reception of data, the core clock signals K<0>, K<1>, K<2> and K<3> should have a precise phase difference without defects.
FIG. 2 illustrates as an example of a DLL circuit to generate such four core clock signals according to a conventional approach. As shown in FIG. 2, a DLL circuit, as an example of a conventional approach, includes a reference loop 10, a phase multiplexer (MUX) unit 20, an interpolation unit 30, a DCC unit 40, a phase detector 50 and a controller 60.
The reference loop 10 equally divides a delay time corresponding to a period T of an external clock signal C,C# as an input clock. In other words, the external clock signal C,C# is delayed through the plurality of delay units, and reference clock signals KR<0,1,2,3,4,5,6,7> are generated. For example, in dividing a period T of the external clock signal C,C# into eight equal parts, one delay unit delays the external clock signal C,C# by T/8. A signal passed through one delay unit is delayed by T/8, and a signal passed through two delay units is delayed by T/4. A signal passed through n-number of delay units is delayed by nT/8 for the external clock signal C,C#, wherein n is a natural number of not less than 1. Thus a plurality of reference clock signals KR<0,1,2,3,4,5,6,7> having mutually different delays are outputted.
The phase MUX unit 20 includes two phase multiplexer (MUX) circuits. The phase MUX circuits are simultaneously controlled by selection control signals PEVEN, EVEN, PODD and ODD. The respective phase multiplexer circuits respectively select two reference clock signals among the reference clock signals KR<0,1,2,3,4,5,6,7> in response to the selection control signals PEVEN, EVEN, PODD and ODD. For example, when a first phase MUX circuit of the two phase MUX circuits selects a first reference clock signal KR<0> and a second reference clock signal KR<1>, a second phase MUX circuit selects a third reference clock signal KR<2> having a phase difference of T/4 from the first reference clock signal KR<0> selected by the first phase MUX circuit and a fourth reference clock signal KR<3> having a phase difference of T/4 from the second reference clock signal KR<1>. This is why the phase MUX circuits are configured to select a clock signal most approximate to the external clock signal C,C# in response to the same selection control signals PEVEN, EVEN, PODD and ODD applied from the controller 60.
The interpolation unit 30 includes two phase interpolators. A first interpolator circuit of the two phase interpolators generates a first interpolation signal, having an optional phase value provided between two selected reference clock signals, in response to interpolator control signals VCNA and VCNB applied from the controller 60. Here, the two selected reference clock signals are, for example, a first reference clock signal KR<0> and a second reference clock signal KR<1> selected by the first phase MUX circuit. A second interpolator circuit generates a second interpolation signal having an optional phase value provided between two reference signals selected by the second phase MUX circuit. Here, the two reference signals being, for example, a third reference clock signal KR<2> and a fourth reference clock signal KR<3>. The first and second interpolation signals are controlled to have a phase difference of T/4.
The DCC unit 40 includes two Duty Cycle Correction (DCC) circuits, and corrects so that duty cycles of the first and second interpolation signals become 50%, thus individually generating a first internal clock signal K0 and a second internal clock signal K1. The phase detector 50 compares phases of the first internal clock signal K0 and the external clock signal C,C#, and applies a detection signal PHADV corresponding to its difference to the controller 60.
The controller 60 includes a Final State Machine (FSM) circuit having a counter and a Digital-to-Analog (D/A) converter circuit, and generates selection control signals PEVEN, EVEN, PODD and ODD and interpolator control signals VCNA and VCNB in response to a detection signal PHADV applied from the phase detector 50, so as to control the phase MUX unit 20 and the interpolation unit 30.
FIG. 3 illustrates a first internal clock signal K0 and a second internal clock signal K1 referred to in FIG. 2. As shown in FIG. 3, the DLL circuit shown in FIG. 2 according to a conventional approach generates two internal clock signals K0 and K1, and takes roles of four transmission core clock signals through rising edges and falling edges of two internal clock signals K0 and K1. In other words, a rising edge time point of a first internal clock signal K0 is used as a first transmission core clock signal K<0>, and a rising edge time point of a second internal clock signal K1 later by T/4 in phase than that of the first internal clock signal K0 is used as a second transmission core clock signal as K<1>. A falling edge time point of the first internal clock signal K0 is used as a third transmission core clock signal K<2>, and a falling edge time point of the second internal clock signal K1 is used as a fourth transmission core clock signal K<3>.
FIG. 4 illustrates another example of a DLL circuit to generate such four core clock signals. Referring to FIG. 4, a DLL circuit as another example of a conventional approach includes a reference loop 110, a phase MUX unit 120, an interpolation unit 130, a delay unit 134, a DCC unit 140, a phase detector 150 and a controller 160.
The reference loop 110 equally divides a delay time corresponding to a period T of an external clock signal C, C# provided as an input clock, and the external clock signal C,C# is delayed through a plurality of delay units, and reference clock signals KR<0,1,2,3,4,5,6,7> are generated. For example, in dividing the period T of the external clock signal C, C# into eight equal parts, one delay unit delays the external clock signal C, C# by T/8. A signal passed through one delay unit is delayed by T/8. A signal passed through two delay units is delayed by T/4. A signal passed through the n number of delay units is delayed by nT/8 for the external clocks signal C, C#, wherein n is a natural number of not less than 1. Accordingly a plurality of reference clock signals KR<0,1,2,3,4,5,6,7> having mutually different delays are outputted.
The phase MUX unit 120 has one phase MUX circuit contrary to the case of FIG. 2. The phase MUX circuit is controlled by selection control signals PEVEN, EVEN, PODD and ODD outputted from the controller 160. The phase MUX circuit selects two reference clock signals from the reference clock signals KR<0,1,2,3,4,5,6,7> in response to the selection control signals PEVEN, EVEN, PODD and ODD. For example the phase MUX circuit selects a first reference clock signal KR<0> and a second reference clock signal KR<1>. This is why the phase MUX circuits are configured to select a clock signal most approximate to the external clock signal C, C# in response to the same selection control signals PEVEN, EVEN, PODD and ODD applied from the controller 160.
The interpolation unit 130 has one phase interpolator circuit. The phase interpolator circuit generates a first interpolation signal having an optional phase value provided between two reference clock signals selected by the phase MUX circuit, in response to interpolator control signals VCNA and VCNB applied from the controller 160. For example, the first reference clock signal may be KR<0> and the second reference clock signal may be KR<1>. The delay unit 134 delays the first interpolation signal by a phase of T/4. The DCC unit 140 includes two Duty Cycle Correction (DCC) circuits. The DCC circuits correct so that the first interpolation signal and a delay signal passed through the delay unit 134 have a duty cycle of 50%, and individually generate a first internal clock signal K0 and a second internal clock signal K1.
The phase detector 150 compares phases of the first internal clock signal K0 and the external clock signal C,C#, and applies a detection signal PHADV corresponding to its difference to the controller 160. The controller 160 includes a Final State Machine (FSM) circuit having a counter and a D/A converter circuit, and generates selection control signals PEVEN, EVEN, PODD and ODD and interpolator control signals VCNA and VCNB in response to a detection signal PHADV applied from the phase detector 50, so as to control the phase MUX unit 120 and the interpolation unit 130.
Exemplary internal clock signals K0 and K1 are shown in FIG. 5. Beneficially, the first and second internal clock signals K0 and K1 shown in FIG. 5 have the same function as those of FIG. 3.
Such DLL circuits according to a conventional approach have an error occurrence possibility because of the use of rising edges and falling edges of the first internal clock signal K0 and the second internal clock signal K1 provided as transmission core clock signals. A clock tree to generate an internal clock signal is lengthened by employing a DCC circuit. Thus, a clock buffer is additionally needed in the DLL circuit and/or on an end of clock tree, and errors may be caused. Furthermore, a DCC circuit has a limitation in correcting a duty cycle of an input clock signal, and it is difficult to generate four transmission core clock signals having a precise phase difference due to errors of the DCC circuit itself, etc.
In addition, in cases such as the DLL circuit shown in FIG. 4, error occurrence may become high when a delay control is not precise in the delay unit. Thus, a DLL circuit to generate transmission core clock signals having a precise phase difference, and to reduce error occurrence as compared with such conventional circuits, is desired.